The present invention relates to a drive control apparatus for controlling driving of a forklift.
Conventionally, forklifts are widely used as industrial vehicles for handling loads (picking up and placing of loads) in plants. Due to its characteristic usage, the driving stability of this type of forklift greatly varies depending on the load state. For example, the center of gravity of the vehicle is shifted and the driving stability varies between a state of carrying no loads and a state of carrying a load. When carrying a load, the driving stability varies depending on the weight of the load, the height of the fork on which the load is mounted, the tilt angle of the fork. Thus, conventionally, a technique for limiting the driving of a forklift in accordance with the load state to improve the driving stability of the vehicle is disclosed in Japanese Laid-Open Patent Publication No. 2001-163597. In the publication No. 2001-163597, the minimum value of acceleration that causes rear wheels to be raised off the ground surface when the vehicle is traveling rearward is computed based on the load state and is set as a permissible acceleration. The actual acceleration of the vehicle is limited not to surpass the permissible acceleration.
Japanese Laid-Open Patent Publication No. 9-24751 discloses a technique for stabilizing the driving of vehicles such as rice transplanters, combine harvesters, and tractors. Japanese Laid-Open Patent Publication No. 9-24751 proposes a technique in which, when a rice transplanter finishes transplantation of rice seedling and starts driving on a road, the engine speed is lowered to a predetermined engine speed upon switching of main clutch from a disengaged state to an engaged state.
The technique disclosed in Japanese Laid-Open Patent Publication No. 2001-163597 is based on the fact that a vehicle becomes unstable during acceleration, and limits the degree of acceleration. In a sudden acceleration of a vehicle, the technique for limiting acceleration according to the publication No. 2001-163597 may not be able to prevent the vehicle from being unstable. That is, in a state where the clutch, which transmits power of the engine, or a driving source, to drive wheels, and in a state where a direction lever for instructing the traveling direction of the vehicle is in a neutral position, if the accelerator pedal is operated, the engine speed is revved up without accelerating the vehicle. In this state, if the clutch is engaged or the direction lever is put in a driving position, the acceleration is abruptly changed and the vehicle is suddenly accelerated. Therefore, in the acceleration control disclosed in Japanese Laid-Open Patent Publication No. 2001-163597, the engine control is not timely performed to deal with such an abrupt change in acceleration. As a result, the vehicle is abruptly started, which is likely to cause the driving of the vehicle at the start to be unstable.
In contract, in Japanese Laid-Open Patent Publication No. 9-24751, the engine speed is temporarily reduced when the main clutch is switched from the disengaged state to the engaged state, so that an abrupt start is suppressed. However, in an industrial vehicle that carries a load such as a forklift, the driving stability of the vehicle varies depending on the load state. Thus, even if the control disclosed in Japanese Laid-Open Patent Publication No. 9-24751 is applied to a forklift, the driving stability at the start is not fully ensured. That is, even if the control in which the engine speed is reduced to a predetermined speed at the time of engagement of the clutch is performed, the engagement of the clutch permits the driving force to be transmitted to the drive wheels. The vehicle travels accordingly. Therefore, in a forklift, in which the driving stability varies according to the load state, the stability may be ensured even if it takes a certain period of time for the engine speed to be lowered in a case where a load is carried at a relatively low position. However, in a case of harsher loading condition, for example, when a load is held at a relatively high position, the stability may not be ensured. As a result, depending on the load state, the reduction of the engine speed cannot be performed sufficiently early (engine control is not sufficiently early). This makes the driving at the start unstable.